Assassin's Creed IV: Black Flag - Fort Dry Tortuga

This fort is easiest if approached from the east side along the southern part of the island to allow you cover until you and almost right next to the forts defenses. As you loop around the corner get in a few broadsides on the southern defenses and you should be good within a single pass.

Once you are on the ground you can enter the fort via the wall to the left of the main gate and then climb the wall to your left and the scaffolding straight ahead. This will allow you to run along the back of the fort and perform an air assassination on the Captain. You can then enter the war room and kill the fort's Commander.

After the fort is cleared you can get the Treasure x2, Animus Fragment x2, and start the Naval Contract, Private Escort.